The present invention relates generally to antennas, and more particularly to micropatch antennas for global navigation satellite systems.
Micropatch antennas are well suited for navigation receivers in global navigation satellite systems (GNSSs). These antennas have the desirable features of compact size and wide bandwidth. Wide bandwidth is of particular importance for navigation receivers that receive and process signals from more than one GNSS. Currently deployed GNSSs are the US Global Positioning System (GPS) and the Russian GLONASS system. Other GNSSs such as the European GALILEO system are planned. Multi-system navigation receivers provide higher reliability due to system redundancy and better coverage due to a line-of sight to more satellites.
Multipath reception is a major source of positioning errors in GNSSs. Multipath reception refers to the reception by a navigation receiver of signal replicas caused by reflections from the complex environment in which navigation receivers are typically deployed. The signals received by the antenna in the navigation receiver are a combination of the line-of-sight signal and multipath signals reflected from the underlying ground surface and surrounding objects and obstacles. Reflected signals distort the amplitude and phase of the received signal. This signal degradation reduces system performance and reliability.
Performance of an antenna over a particular bandwidth is characterized by various parameters, such as the voltage standing-wave ratio (VSWR) and the directional pattern. A parameter that characterizes the multipath rejection capability of an antenna is the down/up ratio
            D      /              U        ⁡                  (          θ          )                      =                  F        ⁡                  (                      -            θ                    )                            F        ⁡                  (          θ          )                      ,where F(θ) is the antenna directional pattern level at an angle θ in the forward hemisphere and F(−θ) is the antenna directional pattern level at the mirror angle −θ in the backward hemisphere. The zenith down/up ratio at θ=90°, denoted D/U (90), is a commonly used parameter.
Multipath effects can be reduced by various antenna structures, such as a large, flat ground plane or a choke ring. These structures, however, increase the size and the weight of the antenna. To reduce dimensions and keep D/U(90) constant as a function of frequency, PCT International Publication Number WO 2004/027920 (published on Apr. 1, 2004) describes a GPS antenna with reduced multipath reception. The bandwidth is sufficient as a function of VSWR, but too narrow as a function of D/U(90).
Many existing antennas for precision GNSS applications were designed and manufactured for installation on geodetic poles or tripods at a particular height above the ground. For some GNSS applications, however, the antenna needs to be mounted on a vehicle. What is needed is a compact antenna that maintains a wide bandwidth and high multipath rejection for different mounting configurations.